The invention disclosed and claimed herein pertains generally to the field of in situ measurement of various parameters which must be determined in order to improve the performance of a towed array. More particularly, the invention pertains to a system for both calibrating and determining the horizontal displacements of the hydrophones of an array, while the array is being towed through an ocean environment. Even more particularly, the invention pertains to a system of the above type which operates by monitoring the responses of discrete hydrophones to locally generated acoustic test signals, the array being linear and the hydrophones being distributed along the length thereof.
The gathering of data in an acoustic environment by means of an array of electrically interconnected hydrophones is of increasing importance, both for commercial and military purposes. Such arrays are usually linear, and comprise a number of hydrophones distributed over a length which is generally in excess of 300 feet. In order to gather data in an acoustic environment comprising a particular ocean area, one end of a linear hydrophone array is usefully coupled to a towing vessel, which then navigates through the area. The individual hydrophones cooperate or interact to provide a beam, which may be steered to determine the bearing from the array of various acoustic sources.
Over a period of time, the detection capabilities of individual hydrophones in a towed array may unpredictably vary. Consequently, to accurately interpret information provided by the array, calibration of some sort must be performed. In the prior art, various techniques are available for calibrating large sections of a hydrophone array. For such calibration, a calibration or test signal of selected frequency is projected to the array so that it is received simultaneously by every hydrophone of the array section. In cases where multiple hydrophones input an acoustic channel, the gross or collective response of the hydrophones to test signal includes the individual responses of defective hydrophones. In a patent application filed with the U.S. Patent and Trademark Office on Nov. 3, 1978 under Ser. No. 957,391, U.S. Pat. No. 4,205,394 issuing thereon on May 27, 1980, the applicant has disclosed a very useful and highly transportable system which falls into the above class of calibration techniques.
In an alternative approach to the calibration of a towed acoustic array, the responses of individual hydrophones of a channel of an array are monitored, hydrophone by hydrophone, to determine their respective sensitivities to a test signal. However, due to the very large number of discrete hydrophones or hydrophone elements in an array, which may have a length on the order of one mile, such calibration technique is extremely tedious. In addition, such calibration would probably have to be performed at a shore installation, so that changes in hydrophone characteristics occurring after deployment of the array at sea would not be included.
In a third calibration approach, the sensitivity of individual hydrophones of an array are indirectly determined, while the array is deployed, by monitoring an acoustic source which is not located at or focussed upon any individual hydrophone. In this approach, multipath effects interfere with sensitivity measurements. As far as is known, the prior art does not provide any direct means for measuring the sensitivity of the individual hydrophones of a linear towed array during array deployment. Yet it is clear that accuracy of data gathering is maximized by calibrating hydrophones thereduring.
In order to employ a linear towed array to determine the bearing of an acoustic source, it is necessary to know the positions of the respective hydrophones of the array, in relation to one another. It is therefore common practice in the art to assume that all of the hydrophones lie on the same line, at known distances therealong. However, such assumption is often invalid, such as when the towing vessel is unable to maintain a straight track through the ocean, or when a portion of the array sinks to a lower depth than another portion thereof. If the above assumption becomes too inaccurate, the beamforming capability suffers and source bearing detection by means of the array may become unacceptably inaccurate.
In his invention, the Applicant discloses a system by which the actual detection capabilities of respective hydrophones lying along a linear towed array may be directly measured while the array is deployed in acoustic data gathering operations. The system of the invention may also be modified to determine variations in array depth and heading along its entire acoustic aperture.